This application claims priority to Japanese Patent Application No. 9-126708 filed May 16, 1997 and Japanese Patent Application No. 9-335283 filed Dec. 5, 1997.
The present invention relates to an active matrix type liquid crystal display apparatus, particularly, to an active matrix type liquid crystal display apparatus of the in-plane switching type, wherein a liquid crystal is driven by generating and applying an electric field nearly in parallel to the surface of the substrate.
Display of information by a liquid crystal display apparatus is performed by applying an electric field to liquid crystal molecules in a liquid crystal layer interposed between substrates to change the orientation of the liquid crystal molecules and to utilize the change in the optical properties of the liquid crystal caused by the change of the orientation of the liquid crystal to produce a display.
An active matrix type liquid crystal display apparatus of a twisted nematic display (TN) type is typical of a conventional active matrix type liquid crystal display apparatus. In the active matrix type liquid crystal display apparatus of a twisted nematic display type, display of information by the liquid display apparatus is performed by applying an electric field nearly perpendicular to the surface of the substrate to the liquid crystal and utilizing the optical rotating power of the liquid crystal to control optical properties of a display. On the other hand, an active matrix type liquid crystal display apparatus of an in-plane switching type has been proposed, for example, in Japanese Patent Laid-Open No. 63-21907.
In the active matrix type liquid crystal display apparatus of an in-plane switching type, display of information by the liquid display apparatus is performed by applying an electric field nearly in parallel to the surface of a substrate using interdigitated electrodes and utilizing birefringence of the liquid crystal to control optical properties of the display. The in-plane switching mode has the advantages of providing a wider viewing angle and lower storage capacitance compared to the conventional twisted nematic type and, accordingly, is a promising technology for the active matrix type liquid crystal display apparatus.
With the recent trend of high speed response of the active matrix type liquid crystal display apparatus, there occurs a display problem in the form of a so-called after image in the liquid crystal display elements, that is, an image retention phenomenon. The term image retention phenomenon, that is, the after image problem, refers to a state wherein there occurs a zone having a very slow response compared to a normal liquid crystal response speed of approximately 50 milliseconds. Occurrence of the after image phenomenon in the conventional twisted nematic type liquid crystal display apparatus is caused by accumulating a direct current charge in a boundary surface of a liquid crystal alignment layer of each pixel to change the effective voltage. In other words, the after image phenomenon is caused by the fact that an electric potential at the time of application of the voltage is not eliminated within the response time and is held in the alignment layer on the pixel electrode or the boundary surface of the liquid crystal alignment layer to change the effective voltage applied to the liquid crystal layer. The relationship between the after image phenomenon and the residual direct current voltage component has been studied, and, at the present time, it is being suggested that the after image phenomenon can be improved by decreasing the residual direct current voltage. Therefore, as the alignment layer of the conventional twisted nematic mode, an alignment layer having a property of hardly accumulating a direct current charge, that is, having less residual direct current voltage component, is required.
On the other hand, the image retention (after image) phenomenon also occurs in a liquid crystal display apparatus of the in-pane switching type, and consequently it causes a problem in that the image quality is decreased by reduction of the dark level, reduction of contrast and the occurrence of gray scale reversal between adjacent pixels, and the productivity is decreased by reduction of yield. The direct current voltage remaining in the pixel electrode, which relates to the after image phenomenon in a conventional liquid crystal display apparatus of the twisted nematic type, also has been measured in a liquid crystal display apparatus of the in-plane switching type. The results show that (1) there is little significant difference in the residual direct current voltage value between a liquid crystal display element producing an after image and a liquid crystal display element not producing an after image, (2) there are some liquid crystal display apparatuses of the in-plane switching type in which the image retention problem semi-permanently continues to cause substantial decrease in the contrast. Further, it has been clarified from an inspection of the orientation of the liquid crystal in an after image and image retention zone that a liquid crystal which rotates from the initial set orientation toward the driven orientation by a very slight angle, does not completely return to the initial orientation. From the above-mentioned points, it can be seen that the after image and image retention phenomenon in the in-plane switching mode is based on a special after image mechanism of the in-plane switching mode which is completely different from that of the conventional twisted nematic mode, and therefore it is necessary to independently solve the special image retention and after image problem of the in-plane switching mode. Hereinafter, the after image is referred to as an in-plane switching (IPS) after image.
An object of the present invention is to provide a high image quality active matrix type liquid crystal display apparatus which has little display unevenness due to image retention by solving the above-mentioned problem in an active matrix type liquid crystal display apparatus using the in-plane switching mode.
Another object of the present invention is to provide a high image quality active matrix type liquid crystal display apparatus which is suitable for mass-production.
An active matrix type liquid crystal display apparatus in accordance with the present invention comprises a pair of substrates, at least one of the substrates being transparent; a liquid crystal layer arranged between the pair of substrates; an electrode structure for generating an electric field having a dominant component parallel to a surface of the substrate in the liquid crystal layer, the electrode structure being formed on one of the pair of substrates; a pair of alignment layers formed on respective surfaces in contact with the liquid crystal layer of the pair of substrates; and a pair of polarizing plates arranged so as to sandwich the pair of substrates, wherein the glass transition temperature Tg of a boundary surface between the liquid crystal layer and the alignment layer, or the glass transition temperature Tg of a surface of the alignment layer is higher than a nematic-isotropic phase transition temperature T(N-I) of a liquid crystal composite forming the liquid crystal layer.
According to another aspect of the present invention, the surface modulus of elasticity of the pair of alignment layer is not less than 1 GPa.
Further, according to still another aspect of the present invention, the glass transition temperature Tg of the alignment layer is not lower than 300xc2x0 C.
It is preferable that the anchoring strength A2 in the azimuthal direction of a surface of the alignment layer to liquid molecules in the boundary surface between the alignment layer and the liquid crystal layer is less than 20 xcexcN/m.
As a method of changing an optical characteristic depending on the orientation of the liquid crystal molecules, it is preferable that a parameter dxc2x7xcex94n satisfies the relation 0.2 xcexcm less than dxc2x7xcex94n less than 0.5 xcexcm using a pair of polarizing plates having polarizing axes crossing each other at nearly a right angle, where An is the anisotropy of refractive index and d is the thickness of the liquid crystal layer.
It is preferable that at least one of the pair of alignment layers formed on the respective substrates is made of an organic polymer of polyamic acid imide group, polyimide group, polyimide siloxane group and polyamide imide group.
It is preferable that a weight average molecular weight of the organic polymer is not smaller than 10,000 and not larger than 300,000.
It is preferable that the alignment layer is made of an organic polymer of dehydration ring closure of polyamic acid composed of diamine compounds expressed by the chemical formula of H2Nxe2x80x94Rxe2x80x94NH2 and tetracarboxylic acid anhydride, total number including bonding group in the mete position and in the ortho position of bonding group of xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94CH2xe2x80x94, xe2x80x94C(CH3)2xe2x80x94, xe2x80x94C(CF3)2xe2x80x94, xe2x80x94SO2xe2x80x94 making molecular axis of the polymer rotatable in R and X in the repeating structure of the organic polymer being less than 3, the tetracarboxylic acid anhydride being expressed by the following chemical formula. 
An inorganic material layer or a photoreactive material layer can be used for at least one of the pair of alignment layers. Particularly, it is effective when the photoreactive material layer comprises an organic polymer containing a polymer and/or an oligomer containing at least one kind of diazobenzene group and derivatives of diazobenzene group, the photoreactive material layer comprises an organic polymer containing a polymer and/or an oligomer containing at least one kind of stilbene group and derivatives of a stilbene group, and orientation control by irradiation of polarized light is used. Further, it is particularly effective in a case where the photoreactive material is an organic polymer containing a polymer precursor curable by irradiating heat or light or radiation rays, or a case where a transparent organic polymer layer having a thickness thicker than the thickness of the alignment layer is interposed between the alignment layer and the substrate.
It is also more effective when a pretilt angle of the liquid crystal layer is not larger than 5 degrees.